Chapter 8 in the Civ3 manual is all about units. But like so many other chapters in the manual, it doesn't really tell you anything beyond how to move them, that more hit points are good, how to load units on ships and send them by airport... arrrgg! That doesn't tell the player anything. As if that wasn't bad enough, the unit-by-unit reference in the back of the book is innaccurate, listing different stats than what actually occur in the game. For that reason, I'm writing this page on what units really do, what they're good for, and anything else I might want to say about them. I'm not covering unique units (UUs) though; those were already mentioned with their respective civs earlier.
Settler (0/0/1): The settler is your most basic unit, since it is the only one in the game that allows you to create cities. They cost 30 shields and decrease the population of the city that build them by two. While I won't get into a detailed description of settlers here, it's pretty fair to say that you want as many of these as soon as you can get them early in the game. More cities means more of everything; it's difficult to have "too many" cities unless you are sticking them insanely close together. Be warned that they cannot attack or defend though, so protect them at all costs!
Worker (0/0/1): These are just as important as settlers in their own way. Workers are the only unit that can improve tiles used by cities, which means connecting them by road or rail, irrgating plains, mining hills, etc. Since most tiles are all but useless until improved by workers, you will need to have lots of these guys all over the place. Most new players suffer from not building enough workers; I suggest one per city as a rough number to shoot for. I'm not going to spend much time discussing workers or settlers here; they are better described when discussing broader civ-wide strategies, so I'm going to give them the short shrift here in the hopes of covering them in more detail when discussing game openings later.
Scout (0/0/2): Expansionist civs are the only ones that can build scouts, which have a number of uses in the early game. At only 10 shields scouts are very cheap, and they are the only unit that gets 2 movement that can be build right from the start of the game (except for the Aztec UU Jaguar Warrior). In order to make the best use from the expansionist trait, you should build a large number of scouts; probably 2-3 for a standard map and about 8-10 for a huge map. They have several goals; first to scout out land so you can decide where to place your cities, secondly to try and make contact with other civs as soon as possible, and lastly to pop as many goody huts as they possibly can. Scouts can be used in wars as well to conduct pillaging operations, but be warned that they cannot attack or defend themselves.
Warrior (1/1/1): The most basic unit that has both attack and defense values. Warriors are all but useless in combat with only one point in both attack and defense; don't go into war with these guys. Instead, warriors should be used for three other purposes. The first is to use them for scouting, in the case of non-expansionist civs that cannot build scouts (warriors, like scouts, cost 10 shields). The second use is for military police early in the game, especially on Emperor/Deity difficulties where happiness in cities can be a real problem. Sticking warriors in cities can keep you from having to up the luxury rate by providing happiness under Despotism, Monarchy, or Communism. Finally, the other use of warriors is to build them cheaply at 10 shields and then upgrade them en masse to swordsmen at 40g each. I have won many a game using this trick, and it's (IMO) one of the best things warriors are good for.
Spearman (1/2/1): These guys are your early defenders - it's as simple as that. Build a lot of these guys because they can be upgraded all the way to mechanized infantry as part of the longest upgrade chain in the game. There's not much to say about them beyond that; don't try to attack with them except as a last resort. Sometimes it may be a good idea to take spearmen along with you when on the offensive in war to defend your units that are going to be doing the attacking; generally though, they should stay in your cities to defend and deter aggression.
Archer (2/1/1): These units are the anti-spearmen, getting an extra point in attack instead of defense and also costing 20 shields. But while spearmen are highly upgradable and useful, archers are not and serve little purpose in the game on Civ3. Since the only unit that archers upgrade to is the weak longbow one, they don't have much staying power. And if you are going to use a slow (1-move) unit to attack, you're much better off going with swordsmen, not archers. The best use for archers is in an early game "rush" with a militaristic civ or in the event that you cannot secure a source of iron and need to attack something. Essentially what I'm saying here is to use these guys only if you have to.
Swordsman (3/2/1): Possessing the highest attack and defensive values of the ancient age, swordsmen are an absolute terror early in the game. If you are going to fight an early war of conquest, these are the guys to use. They do require iron though and are a bit pricey at 30 shields, but the results speak for themselves. Using the warrior -> swordsmen upgrade tactic can be a winning game move when used at the right time. In a pinch, swords can be used both for attack or defense (though they are clearly best as attackers). The only big downside to swordsmen is that they cannot be upgraded at all; you trade ancient age punch for a unit that will be obsolete in the near future. Timing is everything with swords, since you have to have enough to conquer someone else but not so late that they have become obsolete. Use them with care.
Horseman (2/1/2): This is one of the few fast (2-move) units in the ancient age. Horses can be useful on the attack, but you will see a lot of them dying when going up against spearmen in cities. Since they don't automatically retreat anymore when losing(as in pre-1.21f), you have to be careful with them. And 30 shields is a lot to pay for a unit that only gets two attack. Use them as skirmishers (if you know what that means) that range out to guard stacks of swords who are moving on cities. Horses, unlike swords, are very upgradable, so there's no need to feel like you're wasting your shields here.
Chariot (1/1/2): These units are not all that useful in my opinion. They are part of some very complex ICS strategies that involve having tons of cities and whipping these things, then upgrading them to knights, but I personally have little use for them. By the time you're ready to build them, you will likely have the tech Horseback Riding already. Not all that useful unless you completely build a strategy around them - not something I'm willing to do.
Catapult (0/0/1)(bombard 4/1/1): I don't personally like catapults much, but I have seen them save others before so they are definitely not useless. The thing about these things is that they are almost useless on offense - too slow, bombard value too low to do anything against cities - but on defense they can really help you out. When assaulted by a much larger enemy force, catapults can completely save your butt. Just don't take them on offense; keep a couple on your borders, ready to rush to the site of an invasion across your road network. But don't expect to go out and conquer anyone with these babies.
Galley (1/1/3): There only thing you need to know about galleys is that they are the only ship available in the ancient age. They can carry two units, and have a 50% chance of dying if they end their turn in any non-costal waters (until you discover the techs Astronomy and/or Navigation). Use these things to move your units across water; these are NOT fighting units! Naval combat is pretty unimportant in Civ3 though, so just try to keep your galleys alive. They upgrade a long way too, all the way to industrial age transports.
Explorer (0/0/2): These units are unique in that they are the only ones that have the ability to "treat all terrain as roads." This essentially means that explorers have 6 movement points! Although they are useless for their nominal purpose (the whole world has always been explored by the time these guys come along), they are the BEST pillaging units in the game. And at 20 shields each, they are extremely cheap. I like to take one of these units with me when invading enemy territory to see what units are out there in the fog of war. Someone suggested a while ago that explorers should be thought of as "covert operation units", which is a pretty good description in my eyes. Not a useless unit at all.
Pikeman (1/3/1): What can you say about pikes? They are spearmen that cost 10 more shields and have an extra point in defense. Use these as defenders during the first half of the middle ages. Also extremely upgradable, so putting shields into these is not a waste.
Musketman (2/4/1): Muskets pose an interesting dilemma. They are the next step in the progression of defenders after pikemen, but they cost double the amount of shields (60) and provide only one extra point of defense. Sure muskets have an extra point of attack, but you would be crazy to use these things on attack. A simple comparison shows that 2 pikes provide better defense than 1 musketman, so if you really need to focus just on short-term defense, build the 2 pikes over the one musket. At the same time though you will need to upgrade those pikes to rifles eventually, so you may as well make existing ones muskets before that. Muskets do provide great defense (watch them slaughter knights in cities on hills), but at a costly price.
Longbowman (4/1/1): These units are just terrible. They may have 4 attack, but with only one movement point these things are slow, slow, slow. And the one point of defense means that they will get killed very easily if left exposed at all (watch and see how badly the AI uses longbows). If using longbows, you MUST accompany them with either pikes or muskets for defense. But the only reason to ever use them would be in the unfortunate case that you cannot secure iron and horses to build the far-superior knight unit. Longbows actually are not that bad when used defensively, since they have a high attack value for their cheap cost. They can be hugely effective when defending against an attack from a larger or more powerful civ. But don't take these things on offense unless you're really in a pickle. In the expansion, use Medieval Infantry (same cost, higher defense) instead.
Cavalry (6/3/3): Cavs absolutely rule. Their 6 attack will punch right through all pre-industrial defenders and the three movement points they get is a godsend. Cavalry remains the best attacking unit right up until the development of tanks; they can kill rifles and even infantry if artillery is used in supporting bombardment. If you want to read a good description of cavs in action, look at my GOTM8 report on my games page. While normally these units don't arrive until the industrial age has begun, if for some reason the AI goes to the bottom of the tech tree first, you can crush the world with these things simply because the AI is terrible at dealing with 3-move units. Be warned though that this is the end of the horse upgrade tree; any shields put into cavs stay there.
Knight (4/3/2): If cavalry is THE unit of the late middle ages and early industrial age, knights dominate the first 2/3 of the middle ages. When they appear they have the best attack, defense, and movement rates of any unit. THAT's powerful! Knights are costly though at a whopping 70 shields, so a massive horseman -> knight upgrade can be extremely powerful and a game-winning move (I did this in GOTM8, if you want to see an example). Knights will kill spearmen with ease and still hold even with pikes, though they will begin to encounter problems against muskets. For a significant period of time these will be the best units in the game, so don't be afraid to build a lot of them! Shields put into them are not wasted as knights upgrade into the excellent cavalry unit later on. The only difficulty with knights can be getting the needed resources, so make that a priority if you are fighting a conquest game.
Cannon (0/0/1)(bombard 8/1/1): My comments for cannons are the same as for catapults: good on defense, almost useless on offense. They are too slow to keep up with your knight/cavs and have too low of a bombard value to make a big difference in any case. Build them if you need them for defense, but don't expect to take them with you on offense.
Miscellaneous Ships: There are 4 different ships you can build in the middle ages, and none of them are important enough to deserve their own paragraph. Caravels (1/2/3, carries 3 units) are the upgraded version of galleys and can travel in both coastal and sea squares safely. Galleons (1/2/4, carries 4 units) are an upgraded version of caravels that can travel safely in all sea squares and carry more units. These are the only naval units you are likely to be using, just in order to transport your land units across the water. Frigates (2/2/4)(bombard 2/1/2) are essentially useless units since naval combat means nothing in Civ3 and their bombard value is too low to have much success. My advice is to avoid wasting your production on these. Privateers (2/1/4) could be cool since they have "hidden nationality" and may be good for pissing other humans off when multiplayer comes out, but they are utterly useless in the current version of Civ3. 60 shields for a unit with only 2 attack? I'll pass, thank you. Stay away from these at all costs unless you like wasting production.
Wow, that took longer than I thought. Guess I'll have to put the Industrial and Modern Age units on another page.